The Umpqua Community College shooter was obsessed with Satan and documented his devotion to darkness in a manifesto that was recovered from his computer, a source familiar with the investigation tells PEOPLE. (PEOPLE has not received a copy of this document or confirmed its existence.)

In the reported manifesto, which allegedly had “666” scrawled across it, the source says, 26-year-old gunman Chris Harper Mercer wrote in detail about his desire to “serve darkness.”

“The guy did this strictly for satanic purposes,” the source tells PEOPLE. “He did it to become a god in hell. He wants to be evil. That is his goal, to serve Satan.” Mercer allegedly singled out Christians during his massacre, asking people to state their religion and shooting those who identified as Christians in the head.

The source adds: “The only thing I’ve seen that came close to this type of devotion to Satan is the ‘Night Stalker’ in Los Angeles,” referring to Richard Ramirez, a serial killer who murdered at least 13 people over the course of two years in the 1980s.

Mercer, who killed nine people and injured seven more in Roseburg, Oregon, on Thursday, laments his misfortune, writing in the manifesto, “I am going to die friendless, girlfriendless, and a virgin.” He also lists his favorite movies, colors, musicians and songs before ending the manifesto with pentagrams.

“This is absolutely evil,” the source says. “The guy is a Satanist.” Mercer also died on Thursday afternoon after exchanging gunfire with law enforcement officials.

A spokesman for the U.S. Army confirms to PEOPLE that “Christopher Sean Harper Mercer was in service at Ft. Jackson, South Carolina, from Nov. 5 to Dec. 11, 2008, but discharged for failing to meet the minimum administrative standards to serve in the U.S. Army.”

A Lonely Childhood

Gabriel Rivera, 25, who went to school with Mercer at the Switzer Learning Center, a school for students with learning disabilities and emotional issues in Torrance, California, tells PEOPLE the gunman didn’t have many friends.

“Chris didn’t talk to anyone. He was mad that he was at the school and didn’t like anyone,” Rivera says. “He didn’t have a lot of friends, I can’t even think of anyone who he ever was friends with.”

He adds: “He was really moody, like he’d come into class and just put his head down on his desk and sleep or pretend to sleep. Some of the teachers would ignore him if the class was like 10 kids, but other teachers would make him sit up.”

Rivera says that Mercer liked to draw macabre scenes all over his notebooks. “He had this notebook with all these pictures of skulls that he had traced, and they had like fire in their eyes and looked really weird,” he says. “He would sit in class and just draw weird s— like bones or coffins instead of doing the work.”

A neighbor tells PEOPLE the gunman “had some issues.” Julia Winstead, a 55-year-old blind woman who lives in Mercer’s old apartment building in Torrance, California, says that she never met the gunman, but was friendly with his mother. “She seemed pretty flaky,” Winstead says. “She was a high-anxiety person.”

She says the mother was concerned about her son, complaining that he “was in the house a lot and had some issues. She said that the roaches in their apartment were bothering him and that he wanted to go stay in a hotel.”

Winstead went on to say that she’s disturbed to learn that she was living next to Mercer. “It’s freaky knowing you had a neighbor that had those kinds of issues,” she says. “It was surprising and it is not a comfortable feeling, knowing someone like that was so close by.”

Adds Rosario Espinoza, whose mother lived in the same apartment complex as Winstead and Mercer: “I crossed paths with him once or twice but nothing was said. They kept to themselves.”

Jason Gray, the Chief Medical Officer at Mercy Medical Center, confirms to PEOPLE that 10 shooting victims were brought in Thursday to the hospital. The three most seriously injured went to PeaceHealth Sacred Heart Medical Center RiverBend. Of the remaining seven, one was treated and released last night, two treated and released Friday morning and one died at the hospital.